The Inheritance of Inflorescence Types in Hydrangea macrophylla

The inflorescence shape of hydrangeas can be divided into two types, lacecap and mophead (hortensia), according to the position and number of decorative flowers in the inflorescence. We found a mutant that bore a mophead inflorescence in a wild population of H. serrata in the eastern part of Tamba Highlands in Japan on July 6, 1999. This mophead inflorescence was composed of 200 flowers, of which 80% were decorative flowers. Eighty-six other H. serrata plants in the wild population set several decorative flowers only on the periphery of the inflorescence. No plant showed the intermediate phenotype of inflorescence between lacecap and mophead types in the serrata population. These results indicate that the occurrence of the mophead phenotype in lacecap populations is not through the process of a step-by-step increase in the numbers of decorative flowers and that the mutation of inflorescence type is not a quantitative, but a qualitative change. We investigated the inheritance pattern of inflorescence types of hydrangeas using one lacecap cultivar and two mophead cultivars. All F1 progenies obtained from crosses between lacecap and mophead cultivars bore lacecap inflorescences. Segregation into lacecap and mophead types in F2 progenies fitted the expected ratio of 3 : 1. The crossing test between lacecap and mophead types showed that the segregation of inflorescence types was consistent with Mendelian inheritance and that a single major gene controls inflorescence types. Three sets of original cultivars and their mutants were used for comparisons between inflorescence compositions of lacecap and mophead types in order to elucidate the functions of genes controlling inflorescence type. Total flower numbers of inflorescence and numbers of secondary inflorescences on the axes of the primary inflorescences were reduced with mutation of the inflorescence type from lacecap into mophead. The primary inflorescence axis of the mophead type was longer than that of the lacecap type. Further study will be necessary to clarify if these changes are due to another function of the gene controlling inflorescence types. This study revealed the inheritance pattern of inflorescence types of hydrangeas. This finding will be useful for efficient breeding of hydrangeas.